A conventional friction clutch has at least one pressure plate and a reaction plate, each of which is formed with a machined, generally transverse friction surface, together with a clutch friction wheel or friction disc carrying friction liners. The friction liners are gripped between the friction surfaces of the pressure and reaction plates when the clutch is engaged, which is the normal condition of the clutch.
In a clutch for a motor vehicle having an internal combustion engine, the engine torque is transmitted via the pressure and reaction plates, to the friction disc which is mounted on the input shaft of the gearbox for rotation with it. Both the pressure plate and the reaction plate are mounted on the engine crankshaft for rotation with the latter.
The clutch pedal, operated by the driver of the vehicle, actuates a declutching fork, which in turn operates a clutch release bearing for disengaging the clutch. The clutch release bearing acts in either a thrust (or pushing) mode or a pulling mode. In both cases it is engaged on the ends of the fingers of the clutch diaphragm. The clutch also includes a cover plate fixed to the reaction plate, while the pressure plate has a generally transversely oriented back face. The diaphragm bears on the cover plate and on the back face of the pressure plate, so as to urge the latter towards the clutch friction wheel, whereby the friction liners are gripped between the respective friction surfaces of the pressure and reaction plates.
Thus, when the clutch release bearing is engaged on the diaphragm, it cause the latter to pivot and so relieves the pressure plate of the force normally exerted by the diaphragm. In this way, the pressure plate is displaced axially towards the base of the cover plate. Resilient tongues couple the pressure plate to the cover plate, in such a way that the pressure plate and cover plate rotate together but are capable of relative axial movement.
The travel of the clutch bearing during the declutching operation takes place in two stages, namely an approach stage, at the end of which the friction liners come into kissing engagement with the friction surfaces of the pressure and reaction plates; and a modulation stage, during which the liners are progressively gripped between the associated friction surfaces. During the modulation stage, there is rotational friction between the liners and the friction surfaces, which tends to polish the liners and which causes heat to be produced. The clutch may be part of an automatic transmission, in which the operations of engaging and disengaging the clutch are controlled by an actuator which actuates the clutch release bearing, and which is itself governed by a computer in response to, for example, the speeds of rotation of the engine crankshaft and the gearbox input shaft. In an application of this kind, the friction liners may be deliberately polished beforehand, in order to improve absorption of vibration in a predetermined running mode of the engine, corresponding for example to the natural frequency.
Whether or not the clutch is part of an automatic transmission, not only the liners themselves, but also the pressure and reaction plates and the friction discs all tend to become heated.
Moreover, under severe conditions, for example following a series of consecutive hill starts of the vehicle, it is found that the pressure plate assumes a conical shape under the effect of this heating.
It is therefore desirable to provide means for cooling the pressure plate itself, as well as the friction disc with its liners.
U.S. Pat. No. 3,385,412 disclosed cooling of the friction disc and the pressure plate with the aid of a coolant liquid. With such an arrangement, the clutch is no longer a dry clutch. Such an arrangement involving a coolant liquid is costly, and requires considerable modification of the clutch.
In the context of a clutch which operates dry, it has also been proposed to provide the back face of the pressure plate, i.e. the face opposite to the face on which the friction surface of the pressure plate is defined, with a set of ventilating fins whereby the pressure plate, and to a lesser extent the friction disc and its liners, can be cooled.